The invention concerns especially a hydraulic anti-roll system which is known as BMW's “Active Roll Stabilization” (ARS) system, which e.g. has been disclosed and discussed in EP1175307 and EP0992376. In the known system a vehicle's front and rear anti-roll bars or stabilizers each are split in two halve bars, which are interconnected by a hydraulic motor or rotation actuator. Such a prior art hydraulic motor or rotation actuator may have symmetrical input/output behaviour, i.e. the ratio between the value of the hydraulic input and the value of the mechanical output is equal for each direction (sign) of the hydraulic input. Contrary to that, also configurations exist, e.g. known from US2005/0082781, which show an asymmetrical input/output behaviour, e.g. when applying a piston type actuator, comprising a piston, a piston rod and a cylinder. In that case, the ratio between the value of the hydraulic input and the value of the mechanical output will not be equal for each direction (sign) of the hydraulic input because, due to the presence of the piston rod, the effective surface area of the piston at the side of the piston rod is smaller than the piston's surface area at the other side and, in consequence, the (absolute) value of the mechanical output, given a certain (absolute) value of the hydraulic input, will behave correspondingly. The rotation actuators are controlled by an electro-hydraulic control system, commanding those actuators to adjust roll stiffness over a broad range. The ARS is an active suspension system through which the roll angle of the car can be suppressed while taking a curve.
So, a hydraulic roll stabilizer (or anti-roll bar) in fact is a roll or torsion spring whose torsion moment can be controlled by means of a hydraulic rotation actuator. The anti-roll moments of the front and the rear axle are set so that no (or reduced) roll movements of the vehicle chassis occur while taking a curve. For that purpose detectors and an e.g. computerized control system will be necessary, controlling the hydraulic pressure in actuators and thus the moments of the individual anti-roll bars. It may desirable to control the distribution of the anti-roll moments over the front and the rear axle in dependency of the vehicle speed, because this can influence the car's handling properties. At lower speeds the anti-roll moment may be set about equal, which promotes an agile (maneuverable or neutral) vehicle behaviour. At higher speeds a more stable (or understeered) driving character may be desirable. This can be realized by distribution of the anti-roll moments thus that the front axle contributes considerably more than the rear axle. According to the prior art this can be solved by realizing a hydraulic circuit comprising one pressure control module. FIGS. 1a and 1b show two simplified embodiments of the prior art circuitry, which circuitry is disclosed and discussed extensively in EP1175307 and EP0992376.
Turning to FIG. 1a now, the prior art system thus comprises a series connection of two pressure control valves 1 and 2 through which a hydraulic volume flow can be led, supplied by a pump 3, cooperating with a tank 4. A cascaded and controllable pressure Δp1 and Δp2 is created, controlling the pressure in rotation actuators 5 and 6. The pressure of the rear axle actuator 6 is controlled by valve 2 and amounts to Δp2. The pressure of the front axle actuator 5 is controlled by both valves 1 and 2 and amounts to Δp1+Δp2. The rotation actuators 5 and 6 convert these pressures into the desired anti-roll moments. A double direction valve 7 realizes that of the actuators 5 and 6 a first or a second room—via a first or a second terminal A or B respectively—is provided with a controlled pressure when the vehicle takes a curve to the left or to the right. At the front axle a so-called fail-safe valve (not shown in simplified FIG. 1) may be applied which, in case of system failure, blocks the front axle hydraulically, while the rear axle is geared independently without pressure. This results in a safe understeered driving behaviour of the vehicle. Besides, this valve realizes that the oil is circulated nearly without pressure from pump 3 to tank 4 even when the pressure control valves 1 and 2 are closed. The advantage of the prior art circuit is that all available volume flow distributes itself from the pump (during pressure building) to both actuators according to the actual need. Due to this the energy of the pump is always used optimally. To the connections ‘a’ and ‘c’ in the circuit, the vehicle's control means—e.g. a board computer or processor—may be connected, to regulate the valves 1, 2 and 5. Connection ‘b’ may be connected to a pressure sensor, e.g. for feed-back information to the vehicle's control means.
In FIG. 1a the pressure control valves 1 and 2 are connected in series and form together a pressure control module 8 which, in this prior art configuration controls the hydraulic pressure (Δp2) of the rear axle actuator 6 independently while, however, the hydraulic pressure (Δp1+Δp2) of the front axle actuator 5 is always partly dependent of the pressure (Δp2) of the rear axle actuator 6.
The embodiment of the pressure control module 8 is illustrated in the figures which are disclosed in EP1175307 and in FIG. 2 disclosed in EP0992376. In this configuration two pressure control valves—which, relying on the used symbols, might be pressure relief or limitation valves—are connected in series and their common series connection point, their “middle terminal”, is connected to one terminal of the rear axle actuator 6, the other terminal of which is connected with the tank inlet side. The remaining terminals of the series connected valves are connected with the pump's outlet side and the tank's inlet side. In this prior art configuration there is provided only one pressure control module 8, controlling the rear axle' actuator 6, while the front axle actuator 5 is directly—around the pressure control module 8—to the pump outlet and the tank inlet side.
The FIGS. 1, 3, and 4 of EP0992376 disclose an alternative embodiment in which the pressure module 8 comprises a three-way pressure control valve (15) which, relying on the valve symbol in those figures, might be a three way pressure reduction valve. The middle terminal of the three-way control valve is connected to one terminal of the rear axle actuator 6, whose other terminal is connected with the tank inlet side. In this configuration another pressure control valve (14) is provided, which, relying on the used symbol, might be a pressure relief or limitation valve, which is connected with the pump's outlet side and the tank's inlet side and may serve for protecting the system—e.g. the pump—against overpressure. FIG. 1b shows in a simplified way this alternative embodiment of the pressure control module 8, comprising a three-way pressure control valve 9—for the control of the hydraulic pressure of rear axle actuator 6 via terminal I—and a parallel pressure relief valve 10, arranged for safeguarding the pump 3 against overpressure. Moreover, the parallel pressure relief valve 10 serves for controlling the pressure of the front axle actuator 5 and to prevent that the pump does not supply a higher pressure than necessary, thus saving unnecessary energy consumption by the pump 3.
Disadvantageous of the prior art circuit is that it implies a limitation in the mutual control independency of both axes. In the prior art system the pressure of the rear axle is always less than or equal to the pressure of the front axle. This reduces the performance of vehicles controllers which may be arranged to control yaw motions (rotation movements around the vertical axle of the vehicle). For a sportive and agility increasing character of vehicle behaviour, it is often desirable to enter into a curve with oversteer. This can be reached by temporarily generating larger anti-roll moments (or larger hydraulic pressure) at the rear axle. This, however, is not possible with the current prior art (ARS) system.